This invention relates in general to a pneumatic circuit for controlling an air distribution system. More specifically, the invention relates to a mechanism for adjusting the control circuit in a manner to vary the maximum flow rate of conditioned air in the air distribution system.
The heating and cooling of relatively large buildings such as office buildings is normally accomplished by passing conditioned air through ventilating ducts which direct the conditioned air to separate rooms of the building. Individual temperature control for the separate offices or other sections of the building is achieved by controlling the volume of air flow through the duct or through the air outlet which discharges the conditioned air from the duct into the room. Typically, a flow control device is provided in the duct or outlet to regulate the flow of conditioned air to an air diffuser or similar outlet device, thereby controlling the room temperature. This type of air distribution system is generally high in efficiency and low in cost since it utilizes a single large heating or cooling unit to supply several rooms or floors of the building. At the same time, there is no sacrifice in the individual temperature control for each office.
As disclosed subsequently herein, it is possible in this type of air distribution system to provide a pneumatic control circuit that controls the flow rate of conditioned air such that it is virtually independent of the main supply pressure. As a result, the fluctuations that inevitably occur in the supply pressure have no appreciable effect on the flow of conditioned air into the room that is to be heated or cooled. Although such control circuits function well for the most part, the fact that the maximum air flow is constant is sometimes detrimental to the performance of the system. In many instances, it is desirable to provide a field adjustable upper limit on the maximum air flow. It is also desirable to provide an independent factory calibration of the maximum flow rate in order to compensate for any small physical variations that are present in the mechanical components of the system.
The maximum air flow can be made adjustable by installing a needle valve or other constriction in the thermostat line. By adjusting the needle valve, the rate of bleeding of air from the pneumatic relay can be varied, thus varying the flow rate at the equilibrium condition of the circuit. However, needle valves and the like are relatively expensive components that add significantly to the overall cost and are not suitable when independent factory calibration and field adjustment are required. Furthermore, there is a risk of leakage in the low flow calibration line.
Equally significant, restricting the thermostat line results in a shift in the size relationship among the orifices in the lines that connect with the high pressure side of the pneumatic relay. Proper selection of the orifice sizes is important in achieving pressure independent control since the supply pressure is transmitted through different orifices to the opposite sides of the diaphragm in the relay. The effect of the supply pressure is small and tends to cancel itself, and it can be made to exactly cancel itself for all practical purposes by proper selection of the orifices. However, the use of a needle valve or other adjustable restriction effectively changes the size of one orifice relative to the others and disrupts the relationship among the orifices. Consequently, the pressure independence of the control circuit is destroyed and its overall performance suffers accordingly.
As an alternative to the use of a needle valve or other adjustable restriction, the maximum air flow rate can be made adjustable by providing an adjustment of the equilibrium distance between the diaphragm and the control orifice that bleeds pressure from the air bladder. This changes the feedback pressure that must be applied to the relay at the equilibrium condition and thus changes the flow rate of conditioned air from the ventilating duct. In devising a suitable mechanism for effecting such an adjustment, it is important to avoid introducing potential leaks on the high pressure side of the pneumatic relay below the diaphragm. Leakage above the diaphragm is not particularly objectionable because the top side of the diaphragm is vented to atmosphere. It is equally important to avoid applying off-center torques to the diaphragm or torques that are difficult to reproduce. The adjustment mechanism should also avoid the use of expensive seals and the need to twist the connecting tubes when making adjustments. As indicated previously, another requirement of the adjustment mechanism is that it provide for factory calibration and field adjustment on an independent basis.
The present invention satisfies all of these requirements and at the same time provides a simple and effective device for accurately adjusting the maximum flow rate in the air distribution system. In accordance with the invention, the control orifice which controls bleeding of the air bladder is adjustable toward and away from the top surface of the diaphragm. The exhaust tube which terminates in the control orifice is received in another tube for telescoping extension and retraction. The tube assembly is spring loaded and includes an off-center set screw which rides on a cam. Rotation of the cam extends and retracts the exhaust tube to thereby adjust the position of the control orifice relative to the diaphragm. The set screw is used for factory calibration and the cam is used for independent field adjustment, with neither setting altering the relationship among the orifices. The cam has a pointer and an associated scale to indicate its rotative position which corresponds with the percent of maximum air flow at each setting of the cam.